The goal of this research is to test and updated version of the reflex-partitioning hypothesis. This hypothesis provides a rubric for studies that explore the distribution and "strength" of synaptic input to motoneurons from descending pathways, muscle afferents and the recurrent Renshaw-cell pathway. Our approach should contribute insight into the still unresolved issue of the full role played by muscle spindles and tendon organs in motor control. In addition, the experiments have implications for the fields of developmental neurobiology, neuronal recognition and brain and neuromuscular plasticity. The hypothesis states that "the nature and degree of partitioning of synaptic input to motoneurons of the same motor nucleus are associated with differences in the actions and usage of the muscle units supplied by the motoneurons". A physiological corollary of this hypothesis addresses the possibility that there are relative differences in the partitioning of synaptic input to motoneurons of different "type" (i.e., supplying muscle units with different anatomical, biochemical and physiological properties). In addition, an anatomical aspect of the hypothesis accommodates spinal motor nuclei that supply muscles of widely varying design and function. It is hoped that these studies will contribute information on fundamental issues in motor control that have yet to be resolved. Furthermore, it bears emphasis that many neuromuscular diseases involve degeneration of motor axons with subsequent changes in the innervation ratio and territory of motor units (i.e., muscle unit "clumping"). Such changes have implications for motor unit-muscle receptor interactions at both the peripheral and central levels of the nervous system. For this reason, our studies will hopefully contribute to our understanding of diseases such as ALS, adult spinal muscular atrophy, Friedreich's ataxia, and those of Werdning-Hoffman, Kugelberg-Welander and Charcot-Marie Tooth.